Multi-radiator antennas are frequently used in for example cellular networks. Such multi-radiator antennas comprise a number of radiating antenna elements for example in the form of dipoles for sending or receiving signals, an antenna feeding network and an electrically conductive reflector. The antenna feeding network distributes the signal from a common coaxial connector to the radiators when the antenna is transmitting and combines the signals from the radiators and feeds them to the coaxial connector when receiving. A possible implementation of such a feeding network is shown in FIG. 1.
In such a network, if the splitters/combiners consist of just one junction between 3 different 50 ohm lines, impedance match would not be maintained, and the impedance seen from each port would be 25 ohm instead of 50 ohm. Therefore the splitter/combiner usually also includes an impedance transformation circuit which maintains 50 ohm impedance at the common port, i.e. the input port in case of a splitter and the output port in case of a combiner.
A person skilled in the art would recognize that the feeding network is fully reciprocal in the sense that transmission and reception can be treated in the same way, and, to simplify the description of this invention, only the transmission case is described below.
The antenna feeding network may comprise a plurality of parallel coaxial lines being substantially air filled, each coaxial line comprising a central inner conductor at least partly surrounded by an outer conductor with insulating air in between. The coaxial lines and the reflector may be formed integrally with each other. The splitting may be done via crossover connections between inner conductors of adjacent coaxial lines.
In order to preserve the characteristic impedance, the lines connecting to the crossover element include impedance matching structures.
Unpublished patent application SE1551183-5 discloses an antenna feeding network comprising at least two coaxial lines, wherein each coaxial line comprises an central inner conductor and an outer conductor surrounding the central inner conductor, and wherein at least a first inner conductor and a second inner conductor are indirectly interconnected, for example by at least one connector device which engages with the inner conductors. It has been discovered that it may be difficult and/or expensive to manufacture connector devices which achieve the desired performance. The connector devices may be manufactured using for example extrusion, which requires the extruded part to be cut into appropriate lengths, thereby possibly causing burrs or protrusions at the ends thereof. Alternatively, if the connector devices are manufactured using casting, some form of protrusions may result from where the casting mould is divided. The burrs or protrusions may cause air gaps between the inner conductor and the connector device. This may cause degraded high frequency properties since the capacitance in the indirect interconnection is decreased, which may cause losses in the feeding network. Furthermore, if an insulating layer is provided to achieve indirect interconnection, the burrs or protrusions may cut through the insulating layer, regardless if it is provided on the connector device or on inner conductor, thereby causing undesired galvanic contact and passive intermodulation (PIM).